Car shadows on terrain at a lower level

I recently finished a track that runs along the side of a steep mountain, so it features fairly sharp elevation changes. I have found that I sometimes see the shadows of the cars that are running along a section of track that is higher up, as if the road and terrain were transparent.

It's not really a performance problem at all, but it is slightly annoying and certainly not very realistic. Does anybody have any idea how to correct this? I have tried moving the sun so that the shadows shouldn't fall on other bits of road, but I still get this problem.